1. Field of the Invention
The present invention relates to the art of water softening systems. More particularly, the present invention is directed to a method and apparatus for the efficient use of potassium chloride as the regenerant in a water softener.
2. Description of Related Art
A number of different methods and systems are known in the art for softening water. The water softening process involves the replacement of "hard" ions, such as calcium and magnesium, with "soft" ions such as sodium and potassium. Soft water is often desirable because it is less likely to leave deposits on plumbing fixtures.
Water softeners typically utilize an ion exchange material, typically present as a resin bed, to soften water. In the water softening process untreated water is brought into contact with the resin bed where "hard" ions are exchanged for "soft" ions to provide a source of softened water. After prolonged contact with untreated water, however, the capacity of the resin bed to soften water becomes exhausted. When this occurs, the resin bed may be regenerated by exposing it to a brine solution containing the desired "soft" ions, which process restores its water softening capacity.
The brine needed for regeneration may be formed by dissolving in a quantity of water a regenerant salt having the desired "soft" ions. Typical regenerant salts are sodium chloride and potassium chloride. The type of regenerant salt used determines what type of "soft" ions will be present in the softened water. In particular, sodium chloride results in sodium ions being introduced into the softened water, and potassium chloride results in potassium ions being introduced into the softened water.
Many water softeners regenerate the resin bed automatically. In such systems the resin bed is in service most of the time softening water. When the water softener system determines that regeneration is required, it stops softening water and instead regenerates the resin by exposing it to the brine. A number of different methods are known for automatically determining when to initiate a regeneration. Some of these methods are described in U.S. Pat. Nos. 5,544,072 and 4,722,797, which are incorporated herein by reference. Typically, in such methods, regenerations are performed before the resin bed is completely exhausted, in order to ensure that the user does not run out of soft water.
In addition to determining when to regenerate, many systems automatically select the amount of regenerant to be used in a regeneration step. The regenerant is often provided in the form of dry regenerant salt located in a vessel separate from the resin bed, termed the "brine tank." A measured amount of water is introduced into the brine tank in order to dissolve the desired amount of regenerant, forming a brine. Typically, the rate at which water enters the brine tank, the "fill rate," is fixed, so that the fill time determines the amount of water introduced and therefore the amount of regenerant salt dissolved. The brine is then transferred from the brine tank to the resin bed, so that the resin bed is exposed to a known amount of regenerant during the regeneration process. The used brine is then disposed of as waste.
Sodium chloride (NaCl) has been the regenerant salt most commonly used in water softeners. However, the use of potassium chloride KCl) as the regenerant is an attractive alternative. The potassium ions added to soft water from softeners regenerated with KCl are more beneficial to human health as well as to plant life than the sodium ions added to soft water from softeners regenerated with NaCl. The use of KCl as the regenerant also often results in less chloride being present in the waste brine, making its disposal less environmentally damaging.
Most water softeners, however, are designed for NaCl regenerant and lack the flexibility to operate adequately if KCl is used as the regenerant instead. In particular, if KCl is used as the regenerant, the resin bed may become exhausted prematurely, i.e., before it is regenerated. As a result, the user would run out of soft water. The problem becomes more acute as a function of water temperature and softener efficiency, i.e., the colder the water is that is used to form the brine and the more efficiently the water softener uses regenerant salt, the more likely premature exhaustion is.
Moreover, the use of KCl as the regenerant is more complicated than the use of NaCl for a number of reasons. First, in certain operational regimes, namely, when the resin bed is used most efficiently, the resin bed requires a greater amount of KCl than NaCl for regeneration. Second, the solubility of KCl in water is highly temperature dependent, unlike NaCl. In particular, the solubility of KCl in cold water is greatly reduced relative to NaCl. As a result, when cold water is used to form the brine, a greater amount of water is required to dissolve the KCl. Third, the dissolution of KCl in water is significantly endothermic, so that the KCl cools the water as it dissolves, thereby lowering its solubility even more. Finally, KCl dissolves in water at a slower rate than NaCl.
U.S. Pat. Nos. 5,544,072 and 4,722,797 each disclose a method and apparatus for operating a water softener. These references also disclose that either potassium chloride or sodium chloride may be used as the regenerant, but they do not suggest any changes to the water softening method or apparatus depending on whether NaCl or KCl is used. Such changes are required, however, because of the different characteristics of these two salt types. As a practical matter, then, water softeners in accordance with these references do not have the flexibility to be able to use either NaCl or KCl at the option of the user. Moreover, these references do not disclose any way of accounting for the more complicated characteristics of KCl, such as its temperature dependent solubility, in order to use KCl as a regenerant in an efficient and reliable manner.